Low-rank coals such as lignite and subbituminous coal occupy half of the world's coal resources. It is expected to refine or improve such low-rank coals having a moisture content of about 25 to about 65 mass percent and to use them as refined coals having high caloric values. In principle, the low-rank coals are refined by placing the low-rank coal in a heated oil, and evaporating water from the low-rank coal (dewatering).
The present applicant has proposed a method for producing a briquetted solid fuel using a low-rank coal as a starting material in Patent Literature (PTL) 1.
The method for producing a briquetted solid fuel described in PTL 1 includes steps as follows. Initially, a low-rank coal is pulverized to give a pulverized coal in powder form (pulverizing step). The pulverized coal is mixed with a mixed oil containing a heavy oil and a solvent oil to give a slurry (mixing step). The slurry is heated and thereby dewatered to give a dewatered slurry (dewatering step). The solvent oil is separated from the dewatered slurry to give a cake (solid-liquid separating step). The cake is heated to further separate the solvent oil from the cake to thereby give a refined coal in powder form (drying step). The refined coal is combined with the pulverized coal as a moisture source to moisturize the refined coal to give a moisturized refined coal having a moisture content of 3 to 10 mass percent, where the moisturized refined coal is a mixture of the refined coal in powder form and the pulverized coal (moistening step). The moisturized refined coal is briquetted under pressure using a double-roll briquetter to give a briquetted solid fuel, where the double-roll briquetter is equipped with a multiplicity of pockets (concave briquetting molds) on the roll surfaces (briquetting step).
According to the method for producing a briquetted solid fuel described in PTL 1, the refined coal after the drying step is moisturized to give a moisturized refined coal having a moisture content of 3 to 10 mass percent, and the moisturized refined coal is briquetted under pressure. The presence of the moisture allows coal particles to be bonded with each other more firmly. This enables briquetting that gives a briquetted solid fuel having a high strength without using a binder such as starch. Accordingly, the method for producing a briquetted solid fuel described in PTL 1 enables briquetting cost reduction while allowing the briquetted solid fuel to have a strength maintained at satisfactory level.